The postural instability exhibited by older adults increases fall and injury risk, leading to greater morbidity and decreased quality of life. These changes and the resulting impact are even greater for those who become frail in old age. The purpose of this project is to determine the role of static and dynamic control of muscles surrounding the ankle in postural instability, and mechanisms in the brain, spinal motor neurons, and peripheral nervous system that may underlie the postural dyscontrol. We propose to measure 1) the ability to control ankle force during novel tasks, 2) postural sway during static and dynamic tasks, 3) the effects of cognitive load, visuomotor feedback manipulation, and proprioceptive manipulation on force control, 4) the discharge of single spinal motor neurons during force control tasks, and 5) the functional correlates of impaired control - in healthy and frail elderly adults. The information will enhance knowledge on the changes in the nervous system that lead to impaired control of postural stability, and the accompanying changes in physical function. A greater knowledge of the changes that occur in those who become frail will aid in understanding the best interventions to apply to improve quality of life for the growing number of elderly Americans. PUBLIC HEALTH RELEVANCE: Elderly adults who are less steady while standing may have a greater risk of falling and serious injury. This project will determine some reasons that healthy elderly and frail elderly adults have difficulty controlling their ankle muscles and standing posture. The control of ankle muscles will be measured in healthy elderly and frail elderly adults. The ability of the same individuals to control their posture while standing will also be measured, along with balance and mobility. The signals from the spinal cord to the muscle will be measured with electrodes. Greater knowledge of these changes in the nervous system of aging, frail Americans will lead to the application of more effective exercise programs to combat unsteadiness, frailty, and functional disability in the elderly.